To choose the desired sort behaviour, you need to choose the correct LC_COLLATE value. In this case, the standard built in C (POSIX) locale is suitable:

$ sort testcase
aa
a-b
ac
$ LC_COLLATE=C sort testcase
a-b
aa
ac

If you prefer, you can set all the locale settings (thus being more consistent) by setting LC_ALL=C. Since these are environment variables, you can permanently set your sort order, with export LC_ALL=C or similar, in your shell startup script.